356 MEANS OF FEETILISATION. CHAF. X, 



CHAPTER X. 



MEANS OF FEBTILISATION. 



Sterility and fertility of plants when insects are excluded The means 

 by which flowers are cross-fertilised Structures favourable to self- 

 fertilisation Relation between the structure and conspicuousness 

 of flowers, the visits of insects, and the advantages of cross-fertilisa- 

 tion The means by which flowers are fertilised with pollen 

 from a distinct plant Greater fertilising power of such pollen 

 Anemophilous species Conversion of anemophilous species into 

 entomophilous Origin of nectar Anemophilous plants generally 

 have their sexes separated Conversion of diclinous into herma- 

 phrodite flowers Trees often have their sexes separated. 



IN the introductory chapter I briefly specified the 

 various means by which cross-fertilisation is favoured 

 or ensured, namely, the separation of the sexes, the 

 maturity of the male and female sexual elements at 

 different periods, the heterostyled or dimorphic 

 and trimorphic condition of certain plants, many 

 mechanical contrivances, the more or less complete 

 inefficiency of a flower's own pollen on the stigma, and 

 the prepotency of pollen from any other individual 

 over that from the same plant. Some of these points 

 require further consideration ; but for full details I 

 must refer the reader to the several excellent works 

 mentioned in the introduction. I will in the first place 

 give two lists : the first, of plants which are either quite 

 sterile or produce less than about half the full com- 

 plement of seeds, when insects are excluded ; and a 

 second list of plants which, when thus treated, are fully 

 fertile or produce at least half the full complement 



